Mathias Liewald

Effects of Rolling Direction and Lubricant on Friction in Sheet Metal Forming

Lubrication and friction at workpiece-tool interface play an important role in product quality control of sheet metal forming process. Surface microstructures of sheets have a great influence on the development of lubrication films. In order to investigate the effects of the rolling direction of aluminum alloy sheet and lubricant on the friction behavior in sheet metal forming, strip drawing test was used. The sample used was electric discharge texturing (EDT) surface. Lubricants, both with and without additives, were used. The strip drawing tests were performed at angles between the sliding and rolling directions of 0–90 deg. Variations in the sheet surface topography were analyzed by comparing the sheet surface microstructures and its 3D surface parameters before and after the strip drawing test. Results of the strip drawing tests indicate that the kind and amount of lubricant have great influences on friction at the interface, and the lubricant with additives benefits improving the friction behavior between the sheet and the tool. The EDT surface of the aluminum alloy sheet has an anisotropic frictional property during deep drawing process due to different angles between the sliding and rolling directions. When the sliding direction is parallel to the rolling direction, the coefficient of friction has the highest value. When the angle between the sliding and rolling directions increases, the coefficient of friction decreases. The surface microstructure of the sheets after the strip drawing test at different angles between the sliding and rolling directions has been modified, and its 3D surface parameters decrease significantly to a different degree.

Assessment of forming limit stress curves as failure criterion for non-proportional forming processes

The forming limit stress curve (FLSC) is often recommended as failure criterion for the virtual tryout of forming processes which include non-proportional loading. However, parameters influencing position and shape of the forming limit stress curve are not fully known. Up to today it has not been proved if the forming limit curve is strain path-independent, or at least approximately path-independent. In this study the influence of the parameters yield criterion, flow curve extrapolation and level of pre-stretching on the applicability of the FLSC as failure criterion are assessed. The work is performed using the aluminum alloy AA6014 based on forming limit curves for proportional and non-proportional loading published in Werber et al. Key Eng Mater 502–506:71–76, (2012). FLSCs are generated for yield criteria according to von Mises and Hill’48, for flow curve extrapolations according to Ghosh and Hockett-Sherby as well as for an experimentally measured flow curve. In order to be able to assess the influence of the described parameters on the failure criterion based on the FLSC the application of a mean forming limit stress curve (MFLSC) is used. This method is based on the assumption that all FLSCs gained for proportional as well as non-proportional loading map to one single curve. The influence of yield criterion and flow curve approximation on the FLSCs is addressed; the strain path dependency of FLSCs is proved and the forming limit curves for non-linear loading calculated with an assumed mean forming limit stress curve are compared to the experimentally gained forming limit curves.

EcoForge: Energieeffiziente Prozesskette zur Herstellung von Hochleistungs-Schmiedebauteilen*

Kurzfassung In der “Leittechnologie für Morgen – Ressourceneffiziente Prozesskette für Hochleistungsbauteile” (EcoForge) wird eine verkürzte Schmiedeprozesskette für Hochleistungsbauteile entwickelt, die Energie-Einsparungen von > 30 % ermöglicht. Diese Prozesskette wird für hochfeste duktile bainitische Stähle (HDB) optimiert. Dies geschieht, indem unter direkter Ausnutzung der Schmiedewärme unmittelbar an den Schmiedeprozess eine auf den Stahl zugeschnittene Wärmebehandlung vorgenommen wird. Dabei wird die Gefügeumwandlung im Bauteil durch eine neuartige Hochtemperatur-Wirbelstromtechnik während der Abkühlung ermittelt. Die Messsignale werden online erfasst und stehen zur Steuerung der Temperaturführung im Abkühlpfad zur Verfügung. Noch während der Wärmebehandlung, insbesondere der isothermen Wärmebehandlung im Gebiet des Bainits und in ihrem Anschluss, werden weitere Bearbeitungsschritte wie die Heißzerspanung und die Lauwarmumformung vorgenommen. Die Bearbeitungsschritte finden bei Bauteiltemperaturen von ca. 300–500 °C statt. Diese hohen Temperaturen ermöglichen die Bearbeitung des Zielgefüges bei reduzierten mechanischen Belastungen der Werkzeuge. Die erzeugten Mikrostrukturen werden mittels einer neuentwickelten REM-Bildanalyseroutine quantitativ charakterisiert. Simultan zu den experimentellen Untersuchungen wird die gesamte Prozesskette numerisch abgebildet und die Gefügeevolution der Schmiedebauteile im Prozess simuliert.

Approach for Implementation of Short Cycle Stretch Forming (SCS) to Cupping Processes

Short Cycle Stretch Forming (SCS) is a new stretching process patented by the Institute for Metal Forming (IFU) of the University of Stuttgart in 2006 [. It was mainly developed for outer car panels like doors [ and roofs [ with the aim of:

Manufacturing of Composite and Hybrid Materials by Semi Solid Forming

Materials with good mechanical properties and low density are characteristic for lightweight constructions in automotive and aerospace application as well as in the building industry. For this reason, the manufacturing of composite and hybrid materials is in focus of academic and industrial research. Because of the complex and expensive manufacturing process, the application of composite and hybrid materials in many cases is confined to niche and custom-made products. Therefore, the Institute for Metal Forming Technology (IFU) is concerned with the development of new processes for the manufacturing of metal matrix composites and hybrid components by semi-solid forming. Fibre reinforced AlSi-alloys are produced by the application of laminates made of alternating metal matrix layers and carbon fibre fabrics. Hybrid components are manufactured by the integration of higher-strength materials in the semi-solid forming process of aluminium alloys. Here, the main challenge is the integration of the reinforcing components without damaging due to high thermal and mechanical loads that are affecting during the process. These research activities are not only interesting for the mechanical engineering, but also for the civil engineering as this paper will reveal.

Failure Prediction in Sheet Metal Forming Depending in Pre-Straining and Bending Superposition

The automotive industry nowadays, uses numerical simulation systems to determine process safety of car body parts. Forming simulations are usually used to predict local necking and cracks during the deep-drawing operation or to calculate the spring-back behaviour. Furthermore, FEA is also used for optimizing the hemming process. In this contribution, further development and the use of an enhanced failure criterion for the evaluation of flanging and hemming processes are shown. This criterion describes material failure caused by incipient surface cracks on the bending edge keeping the predominant bending load conditions in consideration. The investigations of the bending conditions in this criterion include loads from previous forming operations and geometrical aspects, such as bending radii. The approach presented in this contribution can deliver a more reliable prediction regarding the expected material failure.

Influence of Different Pre-Stretching Modes on the Forming Limit Diagram of AA6014

In order to evaluate the formability of sheet materials forming limit diagrams (FLD) are recorded which represent the values of major and minor strain when necking occurs. FLDs are recorded based on the assumption that exclusively linear strain paths occur. In real forming parts, however, particularly in those with complex shapes, predominantly non-linear strain paths occur which reduce the accuracy of the failure prediction according to a conventional FLD. For this reason forming limits after loading with non-linear strain paths have to be investigated. In this contribution a systematic analysis of the forming limits of a conventional AA6014 alloy after loading with non-linear strain paths is presented. This material is pre-stretched in uniaxial, plane strain and biaxial direction up to several levels before performing Nakajima experiments in order to determine FLDs. During the pre-stretching process as well as during the Nakajima experiment the strain distribution can be measured online very precisely with the optical deformation measurement systems GOM Aramis or VIALUX. The gained curves are compared to the FLD of the as-received material. The results prove a significant influence of the pre-stretching condition on the forming limits of the used aluminum alloy. For a low pre-stretching in uniaxial as well as in biaxial direction the FLDs show a slightly reduced formability while after higher pre-stretching levels the forming limit can be improved such as for biaxial loading after uniaxial pre-stretching. The formability after pre-stretching in plane strain direction was changed. Also, a shift of the FLD depending on the direction of pre-stretching can be observed.

EcoForge: Resource-Efficient Process Chains for High Performance Parts

Classical forging process chains for the production of high performance parts consist of heating, hot forming, machining and heat treatment stages. Especially the last stage comprises numerous energy consumptive heating and cooling procedures. Consolidation of these process chains has a large energy-saving potential and thus can lead to ecological and economic advantages. In addition, the optimization may result in well enhanced local material properties in the final part. Therefore, the collaborative research project “EcoForge: Resource-efficient process chains for high performance parts” has been initiated. Five renowned research institutes aim at optimising the industrially relevant process chain in order to facilitate the resource-efficient production of forgings with increased load bearing capacity. The six subprojects of EcoForge include both, numerical investigations on the microstructure evolution and cooling behaviour of the forged parts as well as numerous experiments which will be carried out in collaboration with the project’s industrial consulting committee, summarising up to 50 relevant participating companies. The joint project is funded by the AiF and BMWi aiming at major areas of technical developments in Germany.

Characterisation of Sheet Metal Formability – A Review and New Approaches

This paper gives a historical overview of techniques of characterising sheet metal material behaviour. First it reviews the evolution of yield criteria in the past, hardening laws and failure-prediction methods based on comprehensive time tables. After that, it demonstrates some shortcomings of todays material characterisation such as the impact of strain induced texture changes on material characterisation. Then the lecture shows further approaches to a more adequate description of material behaviour with regard to biaxial stress conditions and a new failure criterion for sheet metal bending processes, the so called Bending Limit Curve (BLC) concept. It also presents a new testing method for pure shear and simultaneous tension and shear loading as well as further approaches to understand material behaviour.

Forming of Biocompatible Materials in the Semi-Solid State

Semi-solid processing of materials provides advantages of both forging and casting. Experiments with high-melting and biocompatible alloys aiming at a “near-net-shape” production technology recently have been conducted. Advanced trials showed, that processing of such materials by means of semi-solid forming deliver a huge potential for feasible workpiece shapes and drastically reduces machining time and subsequent surface treatment efforts. In contrast to semi-solid forming of aluminium alloys at relatively low temperature levels any processing of high-melting point alloys in the semi-solid state is much more challenging due to higher forming temperature. Commonly used tool materials provoke high wear rates due to wetting, bonding and melting processes which finally result in a very short tool life time. Thus, more apt materials and composites for forming tools and dies which can withstand corrosion, wear, tear and extreme changes in temperatures have to be found. The development of new design concepts for long-living close-to-production tools based on such new materials will be a future goal.